This invention relates to a process for separating finely-divided solids that are suspended in an aqueous medium and, more particularly, to a process for treating a colloidal suspension of clay, alone or in combination with other solid materials, to enhance the settling of the solids.
Clays having a micaceous or platey structure such as bentonite (a montmorillonite clay) acquire the properties of self-suspension, swelling and gelatinizing when they are mixed with water. The clays are advantageously used for these unique properties in many industries. An example of such a beneficial use is in the well drilling industry where bentonite is a component of drilling fluids to suspend the cuttings and carry them out of the well bore.
These same properties of some clays present considerable problems in other industries when it is desired to remove the clays and other materials from suspension in an aqueous medium. For example, many foundries use a sand washing operation that produces as an undesirable by-product a wash water containing on the order of about 0.2 to about 1.2% by weight of suspended bentonite, carbonaceous material and fine sand. The disposal of this waste product has become a problem in view of recent stringent pollution regulations. The use of standard filtering methods and apparatus for separating the finely-divided material from the waste water before discharging or recycling the water is not entirely satisfactory since the fine suspended particles rapidly blind the filter media.
The colloidal behavior of certain clays upon exposure to water presents a very serious problem when hydraulic methods are employed to effect concentration of ore materials containing the clays. These operations, as exemplified by phosphate ore beneficiation processes used in Florida, produce tremendous volumes of aqueous suspensions of ultrafine material that are extremely difficult to de-water. These suspensions are commonly called "slimes". These slimes, which are encountered by metallurgists and mineralogists, are entirely different from the slimes formed by certain microorganisms or fungi such as Myxomycetes slime molds so that treatments for destroying the latter slimes have no application in solving the "slime problem" encountered in the mineral industry.
Mining of phosphate ore in Central Florida is done by an open-pit method. The practice is to strip the overburden to expose the phosphate ore, called the "matrix", for mining. The thickness of the matrix ranges from one to fifty feet, and it is typically composed of about 1/3 phosphate, 1/3 silica sand and 1/3 clay. The matrix is mined with electric-powered walking draglines and the mined ore is hydraulically conveyed from the mining site to the beneficiation plant. In the typical phosphate beneficiation process, the matrix is initially sized and washed to produce a -3/4 inch +16 mesh (Tyler Standard Series) pebble product. The -16 mesh material is then subjected to a desliming operation to remove substantially all of the -150 mesh waste material prior to the beneficiation of the -16+150 mesh material.
The suspension of the -150 mesh waste slime material is continuously transferred to a settling pond built in a mined-out area. The solids slowly settle to produce an upper layer of clarified water that is recycled to the plant for further use, until the settled slimes reach the top of the dam. The mineralogical composition of the slime solids will vary depending upon the location where the matrix was mined. The occurrence of specific minerals in phosphate slimes is somewhat consistent, but the relative amounts of the components vary considerably depending upon the manner in which the sedimentary phosphate rock deposits were formed. Montmorillonite and attapulgite are present in Florida phosphate slimes along with other materials such as fluorapatite, quartz, fluorspar and heavy minerals. The montmorillonite and attapulgite, which are well known for their colloid-like behavior in water, cause the characteristically slow settling property of the slimes.
It is necessary to continuously replace filled dams with newly-constructed ones in view of the slow settling property of the slimes. Slimes that are transferred to a dam or impounding area from a beneficiation plant may contain as little as about 1.5 to 3% solids. The solids content is increased to only about 15% after three months of settling, and from that time on further densification is extremely slow. A state of equilibrium of only about 25% solids is reached after several years of storage. Therefore, the water content of settled slimes is so great that the storage volume required for the slimes is greater than the volume of the matrix that was removed.
The volume of a quantity of slimes does not appreciably decrease even after many years of storage. Evaporation over a number of years forms a thin crust of material containing about 20% water on the surface of the slimes. At this level of moisture, the crust is virtually impervious to the transfer of moisture so that further evaporation from under the crust does not take place. The slime solids cause the formation of an impervious layer that prevents seepage of water through the dams and the bottom of the impounding area. It has been estimated that there are over 1.5 billion tons of phosphate slime solids being stored in dams in Florida.
Considerable effort has been expended to find an economic and efficient method for bringing about the complete and rapid settling of the extremely fine solid materials. A solution to this problem would be of significant benefit since there are phosphate values in the slimes that could be recovered, substantial areas of land are used for storing the slimes, and the storage of the slimes poses an environmental problem. Also, the quantity of water associated with the stored slime solids, which is estimated to be about 4.5 billion tons, has increasing importance since the availability of water in the Central Florida area is becoming more restricted.
Although considerable research has been conducted by the industry, as well as governmental and other organizations, to find a suitable method for de-watering slimes, no completely satisfactory solution has yet been found. The various methods that were considered include conventional filtration, pressure filtration, hydrocycloning, centrifugation, electrical methods such as electrophoretic and electroosmotic methods, and the addition of various chemicals such as flocculants and dispersants to modify the properties of the clay.
Reagents which are known to be flocculants for other systems do not to any significant degree bring about the accelerated settling of slime solids. A report on the evaluation of a variety of flocculating reagents is given in the Bureau of Mines Report of Investigations 6163 entitled "CHEMICAL AND PHYSICAL BENEFICIATION OF FLORIDA PHOSPHATE SLIMES", published in 1963. None of the reagents evaluated were particularly effective in improving the rate of settling of the slimes, and the maximum settling over a 24-hour period was obtained by using 18 to 140 pounds of citric acid per ton of solids. The most rapid initial settling was obtained in 10 minutes with certain high molecular weight polymers, but after 24 hours the settled value was less than that obtained when citric acid was used.